From wowwiki:

Seal of Command procs "per minute" (ppm), rather than per hit - it will proc an average of 7 times per minute. So you get the same amount of procs no matter your weapon speed, but a slower, high damage-per-strike weapon will do more Seal of Command damage per hit, on average, than a faster, low damage-per-strike weapon rated at the same DPS.

For blood elves, it's not so much; they'll be using Seal of Blood. For alliance paladins, they're stuck with Seal of Command, which operates on a procs per minute basis; in other words, no matter how fast or slow your weapon swings, you're still only going to get, on average, a certain number of procs from SoC. Since SoC's damage is based on how much damage your weapon does, a slow, high damage weapon is much preferred.

For both BE and Alliance paladins, Crusader Strike benefits from a slow, heavy weapon, since its damage is also based on weapon damage; however, for the Alliance, it's a much bigger consideration.

Seal of command can only proc i belive 7 times a minute. No matter what speed the weapon is. So the slower harder hit with a seal of command proc the better.

Slow weapons are favored for Paladins using Seal of Command due to the PPM (proc per minute) system the Seal works by.

The game automatically calculates based on your weapon's speed what % hits should proc Seal of Command to average 7 procs per minute. Whether you're using a fast weapon or a slow weapon, you'll can expect only 7 procs (barring good or bad luck).

What gives slow weapons the edge is the extra wallop they pack. Because Seal of Command is not normalized the damage is calculated by multiplying the total DPS by the weapon speed, rather than by the normalization value. Each .1 attack speed less than 3.8 represents 3% less damage from Seal of Command because of this calculation.

The PPM system also means that haste has zero effect on Seal of Command's DPS. You'll still proc only 7 times each minute.

That's why Alliance Retribution Paladins go for slow weapons, and envy the Horde Paladins' Seal of Blood.

It's not so much the speed of the weapon that counts - it's the damage range on your white damage. Seal of Command procs 7(ish) times per minute for 70% of white damage and Crusader strike can be cast up to 10 times per minute for 110% of white damage. This means that if you've got the mana to CS every CD and keep SoC up you'll be doing an average of 15.9 extra white hits worth of holy damage per minute on top of your white damage. If you're ONLY autoattacking, white DPS is all that matters, but as soon as you're using some abilities you need to factor in nearly 16 extra hits a minute from your own abilities - even more if there's a windfury totem from a shaman in the party.

Weapons at the same item level will have the same basic DPS on them, so the slower weapons of a given item level will have bigger damage per swing. Also, attack power contributes a set amount of DPS (AP/14) rather than a set bonus per swing, and that DPS translates to more damage per swing on slower weapons too: whitedamage=weapondamage+weaponspeed*attackpower/14

Also, I've been lead to believe that haste does improve SoC, as the proc rate is based on your weapon speed BEFORE haste is applied. If I'm wrong there, haste will still increase your white output without touching your SoC damage at all. It doesn't affect cooldowns, so it's not going to do anything for your crusader strikes or judgements.